(1) Field of the Invention
The present invention relates to a sheet-through type document reader and an image forming apparatus including the same, and in particular to a technique for reducing a line-shaped noise caused by a foreign object adherent to a reading glass.
(2) Description of the Related Art
Sheet-through type document readers read images while feeding a document onto a scanner. Compared to platen-set type document readers, which read images by a scanner moving along the document, the sheet-through type document readers have the advantages, such as their small sizes, low costs, low undesired sound, and high productivities. Therefore, the sheet-through types are in the mainstream of document reading devices.
In such a sheet-through type document reader, a roller, a guide member or the like rub against documents during the feeding. This frequently produces foreign objects, such as paper dusts. When such foreign objects are adherent to the reading glass, noises in shapes of lines (line-shaped noises) often occur on read images.
To solve this problem, for instance, a technique for vibrating optical parts including a reading glass at a frequency equal to or more than 5 kHz has developed (see Japanese Unexamined Patent Publication NO. 2000-324312). FIG. 1 schematically shows a structure of a document reader pertaining to such a conventional technology. As FIG. 1 shows, a document reader 60 pertaining to the conventional technology feeds a document 603 using rollers 601 and 602, leads the document 603 to an image reading position using guide members 604 and 605. The document 603 pressed against the reading glass 608 by a backup member 607 is illuminated by an exposure lamp 609, its reflected light enters into a CCD (Charge Coupled Device) 612 via a mirror 610 and a lens 611, and thereby the image of the document 603 is read.
The reading glass 608 is supported by a device frame 613 with elastic members 614, and reciprocates in the direction of the vertical scanning, being driven by a driving device 615. The reciprocation cycle of the reading glass 608 is substantially the same as the reading cycle of the CCD 612. By this operation, foreign objects on the reading glass 608 are removed from a reading area of the CCD 612 in the reading cycle, and the line-shaped noise becomes faint and less noticeable.
However, this document reader makes an undesired sound when vibrating at such a high frequency, and therefore it is not suitable for office use.
Also, color readers, which have been enthusiastically developed in recent years, need CCDs for each primary color of RGB. Therefore, a reading area of a color reader is larger than that of a monochrome reader. This makes it difficult to remove foreign objects from the reading area, compared to the case of the monochrome reader.
Further, the line-shaped noise is more noticeable when a color reader is used than when the monochrome reader is used. FIG. 2 is a graph showing a gamma curve used for gamma correction. In FIG. 2, the horizontal axis represents an input density, and the vertical axis represents an output density. The solid line represents a gamma curve used for a color reader (this curve is hereinafter called the “color gamma curve”), and the broken line represents a gamma curve used for a monochrome reader (this curve is hereinafter called the “monochrome gamma curve”).
As FIG. 2 shows, the inclination of the monochrome gamma curve is steep while the input density is an intermediate density, and the inclination is gentle while the input density is low or high. This characteristic produces the effect of making a read image clear by lowering the density of the background image and heightening the density of the character image. The density of a line-shaped noise caused by a foreign object, such as a paper dust, is already low, but becomes even lower by the gamma correction. Also, the gradation reproducibility is low when the density is high. When images around a line-shaped noise at high density are at high density as well, the line-shaped noise is concealed by the images and becomes less noticeable.
On the contrary, regarding the color gamma curve, the lowering of the gradation reproducibility does not depend on the input density very much. Compared to the case of the monochrome reader, the line-shaped noise is more noticeable in the case of the color reader regardless of the density of the line-shaped noise.
Also, when the foreign object moves across the reading area of the CCD 612, it means that the object moves across CCDs corresponding to colors of RGB respectively. Accordingly, the color of the line-shaped noise changes along the direction of the vertical scanning. When this happens, the line-shaped noise is hardly concealed by the images.